USB: xhci: Remove unused HCD statistics code.

CONFIG_USB_HCD_STAT was used in an abandoned patch to track host
controller throughput statistics.  Since CONFIG_USB_HCD_STAT will never be
defined, remove code that can never run.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>